PolicyKit is an application-level toolkit for defining and handling the policy that allows unprivileged processes to speak to privileged processes: It is a framework for centralizing the decision making process with respect to granting access to privileged operations for unprivileged applications. PolicyKit is specifically targeting applications in rich desktop environments on multi-user UNIX-like operating systems. It does not imply or rely on any exotic kernel features.

PolicyKit is used for controlling system-wide privileges. It provides an organized way for non-privileged processes to communicate with privileged ones. In contrast to systems such as sudo, it does not grant root permission to an entire process, but rather allows a finer level of control of centralized system policy.

PolicyKit works by delimiting distinct actions, e.g. running GParted, and delimiting users by group or by name, e.g. members of the wheel group. It then defines how -- if at all -- those users are allowed those actions, e.g. by identifying as members of the group by typing in their passwords.

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PolicyKit vs. polkit

In the development of PolicyKit, major changes were introduced around version 0.92. In order to make the distinction clear between the way the old and the new versions worked, the new ones are referred to as 'polkit' rather than PolicyKit. Searching for PolicyKit on the web will mostly point to outdated documentation and lead to confusion and frustration, e.g. [1]. The main distinction between PolicyKit and polkit is the abandonment of single-file configuration in favour of directory-based configuration, i.e. there is no PolicyKit.conf.

Structure

PolicyKit definitions can be divided into three kinds:

Actions are defined in XML .policy files located in /usr/share/polkit-1/actions. Each action has a set of default permissions attached to it (e.g. you need to identify as an administrator to use the GParted action). The defaults can be overruled but editing the actions files is NOT the correct way (see askubuntu.com for a bad example)

Authorities are defined in INI-like .pkla files. They are found in two places: 3rd party packages can use /var/lib/polkit-1 (though few if any do) and /etc/polkit-1 is for local configuration. The .pkla files designate a subset of users, refer to one (or more) of the actions specified in the actions files and determine with what restrictions these actions can be taken by that/those user(s). As an example, an authority file could overrule the default requirement for all users to authenticate as an admin when using GParted, determining that some specific user doesn't need to. Or isn't allowed to use GParted at all.

Admin identities are set in /etc/polkit-1/localauthority.conf.d One of the basic points of using PolicyKit is determining whether or not a user needs to authenticate (possibly as an administrative user) or not in order to get permission to carry out the action. PolicyKit therefore has a specific configuration for deciding if the user trying to carry out an action is or is not an administrative user. Common definitions are 'only root user' or 'all members of wheel' (the Arch default).

Actions

Each action is defined in an <action> tag in a .policy file. The org.archlinux.pkexec.gparted.policy contains a single action and looks like this:

The attribute id is the actual command sent to dbus, the message tag is the explanation to the user when authentification is required and the icon_name is sort of obvious.

The default tag is where the permissions or lack thereof are located. It contains three settings: allow_any, allow_inactive, and allow_active. Inactive sessions are generally remote sessions (SSH, VNC, etc.) whereas active sessions are logged directly into the machine on a TTY or an X display. Allow_any is the setting encompassing both scenarios.

For each of these settings the following options are available:

no: The user is not authorized to carry out the action. There is therefore no need for authentification.

yes: The user is authorized to carry out the action without any authentification.

auth_self: Authentication is required but the user need not be an administrative user.

auth_admin: Authentication as an administrative user is require.

auth_self_keep: The same as auth_self but, like sudo, the authorization lasts a few minutes.

auth_admin_keep: The same as auth_admin but, like sudo, the authorization lasts a few minutes.

These are default setting and unless overruled in later configuration will be valid for all users.

As can be seen from the GParted action, users are required to authenticate as administrators in order to use GParted, regardless of whether the session is active or inactive.

Authorities

Authorizations that overrule the default settings are laid out in a set of directories as described above. For all purposes relating to personal configuration of a single system, only /etc/polkit-1/localauthority/50-local.d should be used. The authority files are read in alphabetical/numerical order, where later files take precedence, so that one configuration file can be relied upon to overrule another, e.g.

An authorization needs to be preceded by a heading enclosed in square paratheses. The follows an identification with pairs of unix-user or unix-group and the name. Use semicolons to separate the pairs to include more than one user or group. The designating name of the action is the one from the action's id attribute in /usr/share/polkit-1/actions. The three Result-settings mirror those from the action definition. Here we have overruled the default auth_admin setting and disallowed jack and jill from running gparted.

Admin identities

Like the authorities files, configuration works by letting the last read file take precedence over earlier ones. The default configuration for admin identities is contained in the file 50-localauthority.conf so any changes to that configuration should be made by copying the file to, say, 60-localauthority.conf and editing that file.

/etc/polkit-1/localauthority.conf.d/50-localauthority.conf

# Configuration file for the PolicyKit Local Authority.
#
# DO NOT EDIT THIS FILE, it will be overwritten on update.
#
# See the pklocalauthority(8) man page for more information
# about configuring the Local Authority.
#
[Configuration]
AdminIdentities=unix-group:wheel

The only part to edit (once copied) is the right part of the equation: As whom should a user authenticate when asked to authenticate as an administrative user? If she herself is a member of the group designated as admins, she only need enter her own password. If some other user, e.g. root, is the only admin identity, she would need to enter in root's password. The format of the user identification is the same as the one used in designating authorities. The Arch default is to make all members of the group wheel administrators.

Action groups

The actions available to you via PolicyKit will depend on the packages you have installed. Some are used in multiple desktop environments (org.freedesktop.*), some are DE-specific (org.gnome.*) and some are specific to a single program (org.archlinux.pkexec.gparted.policy). The commandpkaction lists alle the actions defined in /usr/share/polkit-1/actions for quick reference.

To get an idea of what PolicyKit can do, here are a few commonly used groups of actions:

ConsoleKit (org.freedesktop.consolekit.policy) actions regulated by PolicyKit include shutting down and restarting, including when other users may still be logged in.

Upower (org.freedesktop.upower.policy) actions regulated by PolicyKit include suspending and hibernating.

NetworkManager (org.freedesktop.NetworkManager.policy) actions regulated by PolicyKit include turning on and off the network, wifi, or mobile broadband.

Limitations

PolicyKit operates on top of the exisiting permissions systems in linux -- group membership, administrator status -- it does not replace them. The example above prohibited the user jack from using the GParted action, but it does not preclude him running GParted by some means that do not respect PolicyKit, e.g. the command line. Therefore it's probably better to use PolicyKit to expand access to priviledged services for unpriviledged users, rather than to try using it to curtail the rights of (semi-)privileged users. For security purposes, the sudoers file is still the way to go.

Documentation

For a more thorough introduction and more advanced features, including globbing, see the man pages of polkit and pklocalauthority: